Transmission electron microscopy has two modes or operation: (1) TEM image observation mode in which a TEM image that is a magnified image of a magnified specimen is observed; and (2) diffraction image observation mode in which a diffracted electron beam image focused at the back focal plane of the objective lens is projected onto the screen by the imaging lens system and observed. The latter mode is referred to as the selected area electron diffraction (SAED) method. The operations of the imaging lens systems in the TEM image observation mode and in the diffraction image observation mode are schematically illustrated in FIGS. 3(a) and 3(b). Shown in these figures are a specimen 1, an objective diaphragm 2, a selected area diaphragm 3, a screen 4, an objective lens OL, a first intermediate lens IL.sub.1, a second intermediate lens IL.sub.2 and a projector lens PL.
FIG. 3(a) shows the operation of the imaging lens system in the TEM image observation mode. In this mode, the objective diaphragm 2 is inserted to improve the contrast of the TEM image. The selected area diaphragm 3 is taken off. The electron beam is converged so as to obtain the desired brightness.
When the electron beam hits the specimen 1, a diffraction image P.sub.1 is focused at the position of the objective diaphragm 2, i.e., at the back focal plane of the objective lens OL. Also, a TEM image Q.sub.1 is focused at the position of the selected area diaphragm 3. The first intermediate lens IL.sub.1 focuses the TEM image at a desired position while taking the TEM image Q.sub.1 focused as the position of the selected area diaphragm 3 as an object plane. The TEM image is subsequently magnified by the second intermediate lens IL.sub.2 and the projector lens PL and focused onto the screen 4. The arrows indicate the TEM images.
FIG. 3(b) illustrates the operation of the lenses in the selected area diffraction image observation mode. In this mode, the objective diaphragm 2 is taken off, and the select area diaphragm 3 is inserted. The electron beam illuminates a wide area of the specimen such that a collimated beam hits the specimen, whereby a clear diffraction image is obtained. The first intermediate lens IL.sub.1 focuses the diffraction image at a given position while taking the diffraction image P.sub.1 as an object plane, the image P.sub.1 being focused at the position of the objective diaphragm 2. This diffraction image is then magnified by the second intermediate lens IL.sub.2 and the projector lens PL and focused onto the screen 4.
The objective diaphragm has been indispensable when the TEM image is observed in the TEM image observation mode. However, in modern TEMs of quite high resolution, the gap between the pole pieces of the objective lens OL is as small as several millimeters. Therefore, it is difficult to insert the objective diaphragm between the pole pieces. Also, if the insertion of the objective diaphragm is possible, it is substantially impossible to bring the position of the objective diaphragm into the back focal plane of the objective lens. As a result, it is difficult to effectively cut scattered electron beams other than the desired diffracted electron beam to obtain a high-contrast and wide field of view TEM image. In order to obtain a high-contrast and wide field of view TEM image, a selected area diaphragm is used instead of an objective diaphragm. Such an electron microscope is disclosed in U.S. Pat. No. 3,629,575. In this electron microscope, the objective lens is very weakly excited so that the diffracted electron beam is focussed at the selected area diaphragm. However, in this electron microscope, since the objective lens is very weakly excited, the spherical aberration coefficient Cs and chromatic aberration coefficient Cc fairly increase. As a result, it is impossible to obtain high resolution image using such electron microscope.
When crystalline specimens such as a metal specimen is observed, it is necessary to frequently switch the mode of operation between the TEM image observation mode and the diffraction image observation mode. For example, in order to photograph one TEM image and one diffraction image having the same field of view as the TEM image, the mode is usually switched 20 or 30 times. As can be understood from the above description, each time the mode is switched, the objective diaphragm must be inserted or taken off and the selected area diaphragm must be inserted or taken off. In this way, the objective diaphragm and the selected area diaphragm must be operated. Hence, the mode switching has been quite cumbersome to perform.